(a) Field of the Invention
The present invention relates to a technique for the T7 promoter-driven expression of a foreign protein, preferably, a human protein in E. coli having no T7 RNA polymerase. More particularly, the present invention relates to use of a vector including a T7 promoter and an N-terminal HA tag sequence for overexpression of a foreign protein in E. coli having no T7 RNA polymerase, E. coli that includes the vector and is able to overexpress the foreign protein in the absence of T7 RNA polymerase, a method for expressing or producing the foreign protein using the E. coli, and a method for analyzing functions of the foreign protein. Further, the present invention relates to a simple, rapid method for evaluating the expression and cytotoxicity of a human protein in E. coli by using the vector.
(b) Description of the Related Art
From physiological, biochemical and genetic perspectives, E. coli (Escherichia coli) is one of the most well characterized bacteria, and is a useful strain for the production of recombinant proteins or their functional studies in biotechnology. The use of E. coli in the production of recombinant proteins has many advantages of high production efficiency and a reduction in time and cost. However, when a vector having a heterogeneous promoter is introduced into E. coli, and components required for expressing the protein do not exist in the endogenous E. coli gene expression system, the target protein is not produced in E. coli. 
Representative examples thereof include the pcDNA and pCS2+ mammalian expression vectors widely used for molecular cloning. These vectors contain the cytomegalovirus (CMV) promoter and the bacteriophage T7 and/or SP6 promoter(s) to drive transcription of genes in mammalian cells and in vitro, respectively. The bacteriophage T7 promoter is known to be much stronger than the E. coli promoter, but it has an extremely high specificity for intrinsic T7 RNA polymerase (T7 RNAP). Thus, when an expression vector utilizing the T7 promoter is transformed into E. coli having lacking T7 RNAP, target protein is hardly expressed. In particular, T7 promoter-driven gene transcription is known to rarely occur in E. coli DH5α and TOP10 strains lacking T7 RNAP, which are widely used in molecular cloning (Melton, D. A., et al., 1984. 12(18): p. 7035-56; Studier, F. W. and B. A. Moffatt, J Mol Biol, 1986. 189(1): p. 113-30; Chamberlin, M., J. McGrath, and L. Waskell, Nature, 1970. 228(5268): p. 227-31; McAllister, W. T., Cell Mol Biol Res, 1993. 39(4): p. 385-91).
Therefore, these vectors have not been used for the expression of the cloned gene in E. coli, but mainly used for increasing the copy number of the plasmid DNA through replication of the vector plasmid DNA in E. coli. Alternatively, for the T7 promoter-driven expression of the cloned gene from the vector, E. coli should be transformed by introduction of a T7 RNAP gene or by insertion of the T7 RNAP gene under the control of the E. coli lac promoter in order to utilize the T7 promoter in E. coli. 
The related art patents, Korean Patent No. 0262867 discloses that T7 promoter-driven expression of recombinant human granulocyte-colony stimulating factor in E. coli is achieved by using E. coli BL21 (DE3) transformed to express T7 RNAP as a host cell and a vector having the lac operator placed downstream of the T7 promoter, and Korean Patent No. 0389378 discloses that T7 promoter-driven expression of CSBP-2 (Cytokine-Suppressive Anti-inflammatory Drug-Binding Protein 2) in E. coli is achieved by using E. coli BL21 (DE3) transformed to express T7 RNAP as a host cell and culturing the E. coli at low temperature. As such, the conventional methods have the problem of requiring E. coli with T7 RNAP for T7 promoter-driven expression of foreign proteins in E. coli. 
If recombinant proteins can be expressed from the mammalian expression vector pcDNA using the endogenous E. coli gene expression system in the absence of T7 RNAP, it is possible to express human genes in a wide range of E. coli strains as well as in mammalian cells, and it is also expected to bring tremendous progress in functional studies of proteins or mass production of industrially useful proteins by means of E. coli. 